The liquid crystal display is widely used as a display of various information processing devices including computers and televisions. In particular, the liquid crystal display of TFT system (hereinafter also referred to as “TFT-LCD”) is widely used with further expansion in the market, and thus further advancement in image quality is desired accompanied therewith. The following description is made using the TFT-LCD as an example, but the present invention is not limited to the TFT-LCD, and is also applicable to passive matrix type LCD, plasma address type LCD and the like, and is generally applicable to all LCD that performs display by sandwiching the liquid crystals between a pair of substrates respectively formed with electrodes, and applying voltage to the electrodes.
The mode that has been most widely used up to now for the TFT-LCD is a so-called TN mode in which the liquid crystal having positive dielectric constant anisotropy are aligned horizontally between the substrates facing each other. The liquid crystal display of TN mode has a feature in that the alignment direction of the liquid crystal molecules adjacent to One of the substrates is twisted by 90° with respect to the alignment direction of the liquid crystal molecules adjacent to the other substrate. In such liquid crystal display of TN mode, an inexpensive manufacturing technique is established. Therefore the liquid crystal display of TN mode is industrially mature, but there is room for improvement in the way that high contrast ratio is difficult to achieve.
On the other hand, a liquid crystal display of a so-called VA mode in which the liquid crystal having negative dielectric constant anisotropy are aligned vertically between the substrates facing each other is disclosed (see e.g., Japanese Laid-Open Patent Publication No. 2000-39610). As described in Japanese Laid-Open Patent Publication No. 2000-39610, in the liquid crystal display of VA mode, the liquid crystal cells hardly exhibit birefringence and optical rotation, and the light passes through the liquid crystal cell without hardly changing the polarized state since the liquid crystal molecules are aligned in a direction substantially vertical to the substrate surface when a voltage is not applied. Therefore, a substantially complete display of black can be realized when a voltage is not applied by arranging the pair of linear polarizing elements above and below the liquid crystal cell so that the adsorption axes thereof are substantially orthogonal to each other. When a voltage is applied, the liquid crystal molecules are inclined and become substantially parallel to the substrate, thereby exhibiting a large birefringence and achieving a display of white. Therefore, the liquid crystal display of VA mode can easily achieve a very high contrast ratio, which is not possible with the TN mode.
However, in the liquid crystal display of VA having the configuration described above, there is room for improvement in the way that the viewing angle is difficult to widen. A substantially complete display of black is achieved in the liquid crystal display of VA mode since the liquid crystal cells hardly exhibit birefringence at the front view and two polarizing elements are completely orthogonal to each other, as described above, but the liquid crystal cells exhibit birefringence and has an apparent retardation at an oblique viewing angle, and the apparent geometric relative relationship of the two polarizing elements are no longer orthogonal to each other, whereby light leakage occurs to lower the contrast ratio and as a result, narrowing the viewing angle. Therefore, a retardation film is often arranged in the liquid crystal display of VA mode in an aim of canceling the extra retardation at the oblique viewing angle of the liquid crystal cell and maintaining the orthogonality of the Cross-Nicol arranged polarizing element at the oblique viewing angle. For example, a technique is conventionally disclosed for widening the viewing angle by arranging the polarizing elements on both sides of the liquid crystal cell aligned vertically, and arranging at least one of a uniaxial retardation film (so-called positive A plate) having an in plane optical axis and where extraordinary ray refraction index>ordinary ray refraction index, a uniaxial retardation film (so-called negative C plate) having an out of plane (film normal line direction) optical axis and where extraordinary ray refraction index<ordinary ray refraction index, or a biaxial retardation film between the polarizing element and the liquid crystal cell (see e.g., Japanese Laid-Open Patent Publication No. 2000-39610, Japanese Laid-Open Patent Publication No. 11-258605, Japanese Laid-Open Patent Publication No. 10-153802, Japanese Laid-Open Patent Publication No. 2000-131693). The uniaxial retardation film having out of plane optical axis and where extraordinary ray refraction index>ordinary ray refraction index is also referred to as positive C plate in the present specification.
A so-called IPS mode is disclosed in which a lateral electric field is applied to the horizontally aligned liquid crystal cell in which liquid crystals are sandwiched between two upper and lower substrates performed with parallel alignment process on the surface and the liquid crystal molecules are rotation operated in a plane substantially parallel to the substrate to perform the display (see e.g., Japanese Laid-Open Patent Publication No. 6-160878). In the liquid crystal display of IPS mode, display is performed by changing the angle formed by the major axis direction of the liquid crystal molecules and the absorption axis of the polarizing element with the liquid crystal molecules constantly being substantially parallel to the substrate, and thus change in birefringence of the liquid crystal cell is small even at the oblique viewing angle, and the viewing angle becomes wide. However, although two polarizing elements are arranged orthogonal (Cross-Nicol) to each other to improve the contrast ratio in the liquid crystal display of IPS mode having the above described configuration, similar to the liquid crystal display of VA mode, the geometric relative relationship of the two polarizing elements are not apparently orthogonal at the oblique viewing angle, and thus there is room for improvement in the way that light leakage occurs in display of black and the contrast ratio lowers. Therefore, in order to improve the lowering of the contrast ratio, arranging the retardation film in the liquid crystal display of IPS mode is also being considered, and for example, the technique of arranging between the polarizing element and the liquid crystal cell an appropriate biaxial retardation film in which the in plane retardation and the retardation in the thickness direction are controlled is known (see e.g., Japanese Laid-Open Patent Publication No. 11-305217).
As described above, (1) maintaining orthogonality of the polarizing elements arranged in Cross-Nicol at the oblique viewing angle, similar to the front view (all modes), (2) canceling the extra retardation of the liquid crystal cell at the oblique viewing angle (VA mode etc.) are important in widening the viewing angle of the liquid crystal display using the pair of polarizing elements having the Cross-Nicol relationship and the liquid crystal cell, where (1) and (2) are conventionally achieved by arranging an appropriate retardation film. Such technique for widening the viewing angle using the retardation film is widely known, but in all conventional techniques, the retardation condition is most suitably designed only at a single wavelength (normally near 550 nm), and thus light leakage occurs in time of display of black at wavelengths other than the designed wavelength, whereby there is room for improvement in the way that coloration phenomenon occurs at the oblique viewing angle.
The conventional liquid crystal display has an essential problem in that the retardation film can be designed only at the single wavelength due to restrictions in types and stacking orders of the retardation film used, the fact that the supporting layer (presently, most general layer is a triacetyl cellulose film=TAC film) for protecting the polarizing element unintentionally has retardation and the like. More specifically explaining, although the wavelength property (wavelength dispersion property) most suitable for the retardation film for achieving (1) or (2) described above differs, the wavelength property cannot be essentially optimized in the prior art due to reasons that the plurality of retardation films of different material are needed to achieve (1) and (2), the retardation film is designed so that (1) and (2) are achieved by the entire assembly of the plurality of retardation films (including TAC film for protecting polarizing element and the like), or the retardation film is designed so as to achieve (1) by actively using the extra retardation at the oblique angle of the liquid crystal cell without achieving (2), and furthermore, the plurality of retardation films are not arranged adjacent to each other. This is because when a plurality of retardation films are used, the effects thereof generally differ depending on the stacked order, and furthermore, the additive property of the retardation of the retardation film is generally satisfied only in extremely limited cases. Moreover, optimizing the wavelength property including the supporting layer (protective film such as TAC film etc.) of the polarizing element is complicating.
In describing one example of a method of designing the retardation in the conventional liquid crystal display, for example, when configuring the liquid crystal display of (first polarizing element)/(TAC film)/(positive A plate a)/(VA mode liquid crystal cell)/(negative C plate b)/(negative C plate c)/(TAC film)/(second polarizing element) using a total of three retardation films of the positive A plate a, the negative C plates b and c, and two polarizing films (configured by polarizing element and TAC film) and the VA mode liquid crystal cell, some of the extra retardation at the oblique viewing angle of the VA mode liquid crystal cell is cancelled, that is, part of (2) is achieved with (negative C plate b)+(negative C plate c)+(protective TAC film of first polarizing element)+(protective TAC film of second polarizing element), and maintenance of orthogonality at the oblique viewing angle of the polarizing elements arranged in Cross-Nicol, that is, rest of (1) and (2) are realized with (positive A plate a)+(rest of extra retardation at oblique viewing angle of VA mode liquid crystal cell).
Therefore, in the prior art described above, the design of the retardation is very difficult, and the most suitable design that takes even the wavelength property into consideration is substantially not possible.